The present invention relates to surgical instruments for the purpose of biting bones and specifically to those of the Kerrison or similar type.
Kerrison type rongeurs are utilized in spinal surgery to remove the laminar bone from the posterior aspect of the spine and to thereby gain access into the spinal canal. The surgeon places the leading edge of the lamina within the open portion of the distal end of the Kerrison rongeur, and then by squeezing the handle causes the moving slide portion of the rongeur to be advanced through that portion of bone to the stationary foot plate thereby severing, or biting out, that portion of bone. The rongeur is then completely removed from the surgical site and passed to the scrub nurse for the removal from the instrument of that removed bone fragment.
To facilitate that necessary function, the foot plate is generally cupped, as is the cutting end of the moveable slide. If only the slide were cupped and the foot plate flat, then the bone would be so compacted into that singular cup as to make it nearly impossible to remove the fragment. However, because the foot plate portion of the Kerrison rongeur is below the lamina and proximate to the dural sac, spinal cord, and nerve roots, there is a compelling need to try to avoid any excessive thickness of the foot plate itself.
Therefore, the foot plate cup is generally not quite as deep as the slide cup and thus, even with the double cup design, the bone fragment tends to be compacted proximally, making its removal nevertheless somewhat difficult. The removal from the instrument of the bone fragment often requires that the nurse use a small rigid hook, or toothed forceps, and often further requires that the physician temporarily relinquish the instrument entirely to the nurse to make such bone removal possible. Once cleaned, the instrument is returned to the surgeon who in returning it to the surgical site must then reorient himself to the task at hand. This sequence must then be repeated over and over again with each bite of bone taken. Typically, such spinal procedures unfortunately require many such bites.
A consideration of the structure and function of the prior art rongeurs, and specifically in regard to the foot plate structure and its requisite thickness, is quite revealing. It would appear that in use the foot plate is subjected to five types of forces.
Consistent with its intended purpose, the foot plate is subjected to, and must withstand, that force necessary to actually cut through the bone, which we shall call the Bone Cutting Force. However, the surgeon has no way of knowing what that force is or even when he has reached or exceeded it. Accordingly, the foot plate is invariably exposed to a second force significantly greater than the Bone Cutting Force which we shall call the Terminal Squeezing Force. The Terminal Squeezing Force occurs after the bone fragment has been cut and is caused by the surgeon generating force in excess of the Bone Cutting Force. This results in the relatively massive slide portion of the instrument being driven with great mechanical advantage against the foot plate.
A third force encountered by the foot plate is a product of the fact that the instrument jaw generally opens to an extent greater than the combined depths of the cups such that the solid bony contents are physically crushed. This is the Bone Crushing Force, and again is additional to the Bone Cutting Force.
A fourth force that may impact upon the foot plate is that which occurs when the jaws of the rongeur encounter an object, which because of its physical structure, is unbiteable. In this situation, while the jaw is still in a relatively open position, again a force greater than the Bone Cutting Force is generated and in this case is then transmitted through the unbiteable object to the foot plate.
The fifth force to which the foot plate is subjected is leverage. When the jaws are not sufficiently sharp, or are worn such that they fail to completely close, then the bone will not be completely cut through, and the surgeon will rock the instrument back and forth to fracture through the remaining bony bridge. In this situation, the angle of the jaw in contact with the leading edge of the lamina becomes the fulcrum point. The foot plate, measuring generally less than one half of an inch in length, is one lever arm, while the remainder of the instrument through the shaft and handle is the other. Since these instruments generally measure on the order of 10 inches or so, the mechanical advantage, or force applied to the tip in a rocking maneuver is on the order of a magnitude of 20 to 1.
The ability to safely withstand repeated exposure to these five forces, and the previously discussed need to cup the innersurface of the foot plate, have in the past, determined the requisite thickness of the foot plate.
Since the prior art rongeurs required removal of the instrument after each cutting procedure, there was little, if any, benefit from use of a power source to operate the rongeur, as a single cutting operation was still all that could be achieved.
Reference is made to U.S. Pat. Nos. 4,722,338 to Wright et al. and 4,777,948 to Wright et al. 4,777,948 discloses a rongeur having a stationery hollow tubular cutting element 28 which may be removably attached to the rongeur. The entire assembly must be disengaged to replace the cutting element. Further, the device is not capable of being a multi bite rongeur, since only a short recess is provided for pulling the severed bone into the hollow cutting tube and the bone is then ejected after each cutting operation, as explained in U.S. Pat. No. 4,722,338 at Col.3, line 10. The collection of the cut bone is not achieved by the hollow cutting element. In fact, the cut specimen could fall into the wound, an unacceptable situation.
Further, in the Wright devices, attempted cutting of an unbiteable object will result in the breakage of a pin, as in existing conventional rongeurs, or breakage of the foot plate.